Organic light emitting devices (OLEDs) are comprised of several organic layers in which one of the layers is comprised of an organic material that can be made to electroluminesce, by applying a voltage across the device. C. W. Tang et al., Appl. Phys. Lett. 51, 913 (1987). Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays. S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995. Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor.
A transparent OLED (TOLED), which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was reported in U.S. Pat. No. 5,703,436, Forrest et al. This TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg--Ag-ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mg--Ag-ITO electrode layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color, red or blue. This colored emission could be transmitted through the adjacently stacked transparent, independently addressable, organic layer, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the red and blue color-emitting layers.
U.S. Pat. No. 5,707,745, Forrest et al, disclosed an integrated SOLED for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. U.S. Pat. No. 5,707,745, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices.
Such devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, OLEDs are comprised of at least two thin organic layers between an anode and a cathode. The material of one of these layers is specifically chosen based on the material's ability to transport holes, a "hole transporting layer" (HTL), and the material of the other layer is specifically selected according to its ability to transport electrons, an "electron transporting layer" (ETL). With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the HTL, while the cathode injects electrons into the ETL. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. These excitons are trapped in the material which has the lowest energy. Recombination of the short-lived excitons may be visualized as an electron dropping from a lowest unoccupied molecular orbital (LUMO) to a highest occupied molecular orbital (HOMO), with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism.
The materials that function as the ETL or HTL of an OLED may also serve as the medium in which exciton formation and electroluminescent emission occur. Such OLEDs are referred to as having a "single heterostructure" (SH). Alternatively, the electroluminescent material may be present in a separate emissive layer between the HTL and the ETL in what is referred to as a "double heterostructure" (DH).
In a single heterostructure OLED, either holes are injected from the HTL into the ETL where they combine with electrons to form excitons, or electrons are injected from the ETL into the HTL where they combine with holes to form excitons. Because excitons are trapped in the material having the lowest energy gap, and commonly used ETL materials generally have smaller energy gaps than commonly used HTL materials, the emissive layer of a single heterostructure device is typically the ETL. In such an OLED, the materials used for the ETL and HTL should be chosen such that holes can be injected efficiently from the HTL into the ETL. Also, the best OLEDs are believed to have good energy level alignment between the HOMO levels of the HTL and ETL materials.
In a double heterostructure OLED, holes are injected from the HTL and electrons are injected from the ETL into the separate emissive layer, where the holes and electrons combine to form excitons.
Various compounds have been used as HTL materials or ETL materials. HTL materials mostly consist of triaryl amines in various forms which show high hole mobilities (.about.10.sup.-3 cm.sup.2 /Vs). One common HTL material is 4,4'-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (.alpha.-NPD), having the structure: ##STR1##
There is somewhat more variety in the ETLs used in OLEDs. A common ETL material is aluminum tris(8-hydroxyquinolate) (Alq.sub.3), having the structure: ##STR2##
Other common ETL materials include oxadiazole, triazole, and triazine.
A number of technological challenges related to OLEDs need further attention, including increasing device lifetime and developing OLEDs that emit bright, saturated colors. Typically, the broad electroluminescence (EL) spectrum of OLEDs results in unsaturated emission colors which can be narrowed using, for example, an absorption filter or microcavity. Unfortunately, these methods can lead to a reduction in the OLED quantum efficiency or a strong angular dependence of the emitted color. It is therefore desirable to develop an OLED which emits a saturated color without the assistance of such external filters. Achievement of saturated and bright red OLEDs has proven to be particularly difficult.
Dyes have been used as dopants in the emissive layers of OLEDs to affect the wavelength and increase the efficiency of light emission. Molecules of the host transfer excitons to molecules of the dye through a non-radiative process. The exciton then recombines on the dye, and emits a photon having a wavelength characteristic of the dye, as opposed to the host. Several of these dyes are polar molecules, ie., are molecules having a significant dipole moment, such as DCM1 and DCM2. DCM1 has a molecular structure represented by the formula: ##STR3##
DCM2 has a structure represented by the formula: ##STR4##
DCM2 has been described as a red emitting chromophore useful in OLED applications. C. W. Tang et al., Electroluminescence of doped organic thin films, J. Appl. Phys. 65, 3610 (1989). Indeed, OLEDs based on Alq.sub.3 doped with DCM2 are shown to exhibit very high brightness. Tang et al. also observed that the emission of Alq.sub.3 doped with DCM1 undergoes a spectral shift towards higher wavelengths as the concentration of DCM1 is increased, and attributed this spectral shift to excimer formation. Id. Tang et al. also observed that, with increasing concentration of DCM1, the efficiency of the emission at first increases, and then decreases.
DCM2 has also been described as showing promise as a laser material. M Berggren et al., Light amplification in organic thin films using cascade energy transfer, Nature 389, 466 (1997).
In studies of solutions of quinacridone in polar solvents, J. Kalinowski et al., Electroabsorption study of excited states in hydrogen-bonding solids: epindolidone and linear trans-quinacridone, Chem. Phys. 182, 341 (1994), spectral shifting has been attributed to hydrogen bonding in solution.
A red shift in the emission of DCM1 has been observed for DCM1 in polar solvents. M. Martin et al., Ultrafast intramolecular charge transfer in the merocyanine dye DCM, Chem. Phys. 192, 367 (1995).